Testing and Diagnosis of High Voltage and Extra High Voltage Power Cables with Damped AC Voltages

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Testing and Diagnosis of High Voltage and Extra High Voltage Power Cables with

Damped AC Voltages

Cichecki, P DOI 10.4233/uuid:f50c2129-6771-468b-aa3c-7c1fdac4e425 Publication date 2018 Document Version Final published version

Citation (APA)

Cichecki, P. (2018). Testing and Diagnosis of High Voltage and Extra High Voltage Power Cables with Damped AC Voltages. https://doi.org/10.4233/uuid:f50c2129-6771-468b-aa3c-7c1fdac4e425

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i

Testing and Diagnosis of High Voltage and

Extra High Voltage Power Cables with

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Testing and Diagnosis of High Voltage and

Extra High Voltage Power Cables with

Damped AC Voltages

Proefschrift

ter verkrijging van de graad van doctor

aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. dr. Ir. T.H.J.J. van der Hagen,

voorzitter van het College voor Promoties,

in het openbaar te verdedigen op maandag 10 december om 15:00 uur

door

Piotr Cichecki

elektrotechnisch ingenieur

geboren te Garwolin, Polen

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Promotor: Prof. dr. J.J. Smit

Composition of the doctoral committee:

Rector Magnificus

chairman

Prof. dr. J.J. Smit

Delft University of Technology

Independent members:

Prof.dr. R. Ross

Delft University of Technology

Prof.dr.ir. P. Bauer

Delft University of Technology

Prof.dr.ing. S. Tenbohlen

Universität Stuttgart

Prof.dr.ing. R. Plath

Technische Universität Berlin

Prof.dr.hab.ir. E. Gulski

Poznan University of Technology

Dr. A. Rodrigo Mor

Delft University of Technology

This research was financially and technically supported by the KSANDR

foundation (The Netherlands) and the ONSITE HV Group (Switzerland)

The cover page was designed by:

Print Point S.C.

Warsaw, Poland

Printed by: Print Point S.C.

Copyright © 2018 by Piotr Cichecki

All rights reserved

ISBN: 978-83-952726-0-8

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S

UMMARY

The thesis focuses on on-site testing and diagnosis of transmission power cables circuits. Based on the application of methods for on-site voltage generation and its use for advanced diagnosis, comprehensive testing and diagnostic procedures have been investigated in this thesis.

The described method utilises damped AC voltages (DAC) for testing purposes and sensitive partial discharge (PD) as well as dielectric loss measurements. As the application of this technique is gaining more and more popularity, the practical issues related to test procedures, diagnostic parameter limits and the general usage experience can be evaluated. Practical applications of the damped AC method including insulation diagnosis are based on investigations of HV cables in the laboratory as well as in the field on-site. Transmission cables in the range of 50 kV up to 230 kV were tested with the DAC method. With regard to cross-linked polyethylene (XLPE) and impregnated paper cable insulation, a comparison of results of laboratory and field experiments has been performed. Failure related defects which are traceable and detectable through PD are described in order to understand the feasibility of the on-site application of DAC voltages for condition assessment of HV/EHV power cables. The thesis shows that DAC test voltages provide a suitable and effective method to detect and localise PD related ageing or installation defects.

Due to the fact that cable joints in newly installed circuits are the weakest links, modern slip-on joints with defects were investigated with continuous AC and damped AC (DAC) energising. Differences in PD amplitude, PD intensity, phase resolved PD patterns (PRPD) characteristics between AC and DAC energising voltages were analysed. The effectiveness of DAC systems in detection of PDs in field conditions was analysed in acceptance (after-lying) tests of newly installed XLPE cables. Practical aspects of DAC voltage level selection, number of DAC voltage excitations and complete test procedures were verified on 110 kV up to 230 kV cables circuits. Issues regarding background noise and disturbance filtering are explained. The interpretation of test results and examples of TDR (Time Domain Reflectometry) analysis are discussed.

To find out a relation between ageing status and dielectric loss values, samples of impregnated paper insulation have been artificially aged in laboratory conditions. Accelerated ageing took almost 8 months at a temperature of 1000C. During this process dielectric loss was measured at different voltages to find the connection with insulation thermal degradation at the various electrical field stressing.

Finally, to support asset management decisions about aged paper insulated cable circuits, life consumption and future life models for paper insulated power cables have been developed. These models combine input data from operational conditions, e.g. ground temperature, cable temperature with dissipation factor measurements using the DAC method. Our developed algorithm may help in selecting the optimum cable load in order to extend insulation life time and avoid accelerated ageing which may improve cable reliability.

The DAC method investigated in this thesis can be used for fast and reliable PD diagnosis of HV and EHV cables according to voltage levels as described in the IEC standard. The compact size of DAC systems and short time necessary to set up the system and to the execute the test is a huge benefit during on-site testing. Information provided by the DAC method allows avoidance of new cable failures and shows the actual insulation condition of in-service cables.

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S

AMENVATTING

Dit proefschrift heeft de focus op on-site testen en diagnosticeren van hoogspanningskabel verbindingen. In deze thesis zijn geavanceerde test en diagnostische procedures voor hoogspanningskabels onderzocht met behulp van een methode, waarbij op locatie hoogspanning wordt opgewekt en tevens diagnostiek kan worden toegepast.

De beschreven methode past gedempte AC spanning (DAC) toe voor test doeleinden met gevoelige deelontladingsmetingen alsmede diëlektrische verliesmetingen. Doordat de populariteit van deze testtechniek steeds meer toeneemt, kunnen de praktische zaken met betrekking tot de test procedures, diagnostische parameter limieten en algemene gebruikerservaring worden geëvalueerd. Praktische toepassingen met de DAC methode inclusief isolatiediagnose zijn gebaseerd op metingen aan hoogspanningskabels, zowel in het laboratorium als op locatie in het net. Transmissiekabels in de spanningsklasse van 50 kV tot en met 230 kV zijn getest met de DAC methode.

Met betrekking tot vernet polyethylene (XLPE) en geïmpregneerd papier/olie kabelisolatie is een vergelijking van laboratorium en veldmetingen gemaakt. Fout gerelateerde defecten, die traceerbaar en detecteerbaar zijn met deelontladingen (PD), zijn beschreven om de haalbaarheid vast te stellen van de testmethode met DAC spanningen voor conditiebepaling van HV/EHV hoogspanningskabels in veldapplicaties Deze thesis concludeert dat een DAC test spanning een bruikbare en effectieve methode oplevert om deelontladingen te detecteren en te lokaliseren, die gerelateerd zijn aan veroudering of installatie defecten.

Omdat kabelmoffen in nieuw geïnstalleerde verbindingen de zwakke schakels zijn, zijn moderne opschuifmoffen met defecten onderzocht met zowel continue als gedempte AC spanning. Verschillen in PD amplitude, PD intensiteit, fase resulterende PD patroon karakteristieken tussen AC en DAC (PRPD) spanning zijn geanalyseerd. The effectiviteit van DAC systemen in het detecteren van deelontladingen in veldcondities zijn geanalyseerd in acceptatie (na-installatie) testen van nieuw geïnstalleerde XLPE kabels. Praktische aspecten, zoals DAC spanningskeuze, aantal DAC spanningspulsen en complete testprocedures zijn geverifieerd aan 110 kV tot en met 230 kV kabelverbindingen. Problemen met achtergrondruis en ruisfiltering zijn toegelicht. De interpretatie van test resultaten en voorbeelden van time domain reflectrometry analyses zijn behandeld.

Om een relatie te vinden tussen de verouderingsconditie en de waarde van de diëlektrisch verliezen, zijn samples van geïmpregneerd papierisolatie kunstmatig verouderd onder laboratorium condities. Versnelde veroudering duurde bijna 8 maanden op een temperatuur van 100⁰C. Gedurende dit proces zijn diëlektrische verliezen voor verschillende spanningen gemeten om een relatie te vinden met thermisch veroudering van de isolatie onder de verschillende elektrische belastingen.

Tenslotte, om de asset management beslissingen te ondersteunen bij verouderde kabels met papierisolatie, zijn levensduurconsumptie en levensduurvoorspellingsmodellen voor papiergeïsoleerde kabels ontwikkeld. Deze modellen combineren gegevens van operationele condities, zoals bijvoorbeeld, grondtemperatuur, kabeltemperatuur, met de dissipatiefactor metingen met behulp van de DAC methode. Ons ontwikkelde algoritme kan helpen in het selecteren van de optimale kabel belasting om zo de isolatielevensduur te verlengen en om versnelde veroudering tegen te gaan, hetgeen de kabel betrouwbaarheid mogelijk zal vergroten.

De onderzochte DAC methode in deze thesis kan worden gebruikt voor snelle en betrouwbare PD diagnoses aan HV en EHV kabels op spanningsniveaus overeenkomstig de IEC standaard. De compacte grootte van het DAC systeem, de benodigde tijd om het systeem op te stellen en de tijd om de test uit te voeren zijn een groot voordeel gedurende netwerktesten. Informatie verkregen met de DAC methode

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T

ABLE OF

C

ONTENTS

Summary ... vii

Samenvatting ... viii

Table of Contents ...ix

1. Introduction ... 1

1.1 (E) HV Power Cables-underground transmission network... 2

1.2 After-laying testing and condition assessment of transmission cables ... 4

1.2.1 New cable systems ... 6

1.2.2 Service aged cable systems ... 7

1.3 On-site testing methodology ... 10

1.4 Asset management for transmission cables ... 16

1.5 Objectives of present study ... 18

1.6 Structure of the thesis ... 19

2. Transmission power cable accessory installation and assembling related defects ... 20

2.1 Accessory assembly ... 20

2.1.1 Assembling process ... 21

2.1.2 Electric field distribution in cable system ... 22

2.1.3 Electric field enhancement ... 26

2.2 Typical installation and assembly defects ... 28

2.2.1 Voids, contaminations on the dielectric surface ... 29

2.2.2 Dielectric surface mechanical damage ... 30

2.2.3 Remaining semi-conductive screen ... 31

2.2.4 Missing semi-conductive screen ... 31

2.2.5 Damages to the slip-on stress cone ... 32

2.2.6 Incorrect joint connector assembly ... 33

2.2.7 Imperfect sealing of joints and terminations ... 33

2.3 Installation and assembly defects symptoms ... 34

2.4 Conclusions ... 35

3. Transmission cables ageing related defects ... 37

3.1 Ageing of oil impregnated cable insulation ... 38

3.1.1 Cellulose chains breaking (de-polymerization/scission) ... 38

3.1.2 Water content (hydrolysis) ... 39

3.1.3 Gas content (gasification) ... 40

3.1.4 Electrical treeing/tracking ... 41

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3.2 Ageing of XLPE cable insulation ... 43

3.2.1 Chemical decomposition ... 44

3.2.2 Physical decomposition ... 45

3.2.3 Electrical erosion/treeing ... 45

3.3 Mechanically induced defects (all types of insulation) ... 47

3.3.1 Mechanical defects due to soil/ground movements ... 47

3.3.2 Thermo-mechanical related defects ... 48

3.4 Defects symptoms ... 50

3.5 Conclusions ... 51

4. Damped AC ... 52

4.1 Damped AC resonance ... 53

4.1.1 Mathematical model ... 55

4.2 DAC testing voltage ... 57

4.2.1 DAC voltage test procedure ... 58

4.2.2 Voltage withstand test ... 61

4.3 Partial discharge measurements using DAC ... 64

4.3.1 IEC 60270 PD detection with DAC ... 64

4.3.2 DAC circuit calibration ... 67

4.3.3 PD parameters measured with DAC (according to IEE400.4) ... 70

4.4 Dielectric loss estimation using DAC ... 78

4.5 Conclusions ... 80

5. Comparison of AC and DAC voltages for defects detection by PD... 81

5.1 Defects for the laboratory investigation ... 82

5.1.1 Electric field distribution for defects 1, 2, 3 ... 85

5.1.2 Effect of the voltage frequency on the electric field and inception voltage in the bounded cavity ... 89

5.2 Description of the test setup... 91

5.2.1 AC energizing method and IEC60270 PD detection... 92

5.2.2 DAC energizing and IEC60270 PD detection ... 92

5.3 Voltage withstand tests after cable installation in the lab ... 95

5.4 Missing outer semi-conductive screen (Defect type 1): 50 Hz AC vs. DAC ... 96

5.4.1 PDIV delay time at 50 Hz AC ... 96

5.4.2 PD magnitude at 50 Hz AC... 97

5.4.3 PRPD patterns at 50 Hz AC ... 98

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5.4.6 PRPD patterns at DAC ... 103

5.5 Extra semi-conductive screen in the joint (Defect type 2): 50 Hz AC vs. DAC ... 105

5.5.1 PDIV delay time at 50 Hz AC... 105

5.5.2 PD magnitude at 50 Hz AC ... 106

5.5.3 PRPD patterns at 50 Hz AC ... 107

5.5.4 PDIV delay time at DAC ... 108

5.5.5 PD magnitude at DAC ... 110

5.5.6 PRPD patterns at DAC ... 111

5.6 Investigation: Electrode-bound cavity (Defect type 3): 50 Hz AC vs. DAC ... 114

5.6.1 PDIV delay time at 50 Hz AC... 114

5.6.2 PD magnitude at 50 Hz AC ... 115

5.6.3 PRPD patterns at 50 Hz AC ... 117

5.6.4 PDIV delay time at DAC ... 118

5.6.5 PD magnitude at DAC ... 119

5.6.6 PRPD patterns at DAC ... 120

5.7 Conclusions ... 123

6. On-site testing and diagnosis of transmission cables with DAC voltages ... 125

6.1 Case study 1: Diagnosis of a newly installed 150 kV XLPE cable after breakdown ... 127

6.1.1 PD vs. Test voltage characteristics ... 128

6.1.2 PRPD patterns analysis ... 129

6.1.3 TDR traces ... 130

6.1.4 PD mappings ... 131

6.2 Case study 2: Testing and diagnosis of a newly installed 220 kV XLPE cable ... 132

6.2.1 PD vs. Test voltage characteristics ... 132

6.2.2 PRPD patterns analysis ... 133

6.2.3 TDR traces ... 133

6.2.4 PD mapping ... 136

6.2.5 Voltage Withstand Diagnosis ... 137

6.3 Case Study 3: Diagnosis of the service-aged transmission cables. ... 137

6.3.1 Procedures for onsite diagnosis of service aged cables with DAC ... 140

6.3.2 PD vs. Test voltage (50 kV Circuit no. 4) ... 143

6.3.3 PRPD patterns (50 kV Circuit no. 4) ... 144

6.3.4 TDR traces (50 kV Circuit no. 4) ... 146

6.3.5 PD mapping (50 kV Circuit no. 4) ... 146

6.3.6 Dielectric loss (50 kV Circuit no. 4) ... 147

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6.4 Overview of the diagnostic results of 13x50 kV oil-filled cables ... 150

6.5 Overview of the diagnostic results of 9x150 kV external gas-pressurised cables ... 153

6.6 Conclusions ... 156

7. Life time estimation of service aged paper-oil insulated cables ... 159

7.1 Laboratory investigation on ageing phenomena of oil-impregnated paper cable insulation .... ... 160

7.2 Insulation life consumption (ILC) calculation ... 168

7.3 Operational life consumption (OLC) calculation ... 169

7.4 Total Life Consumption (TLC) calculation ... 170

7.5 Future life profile calculation ... 171

7.6 The further cable lifetime estimation-model application on service aged 150 kV cables. ... 172

7.7 Conclusions ... 178

8. Conclusions and future work ... 179

Appendixes ... 182

List ... 198

List of publications... 206

Acknowledgment ... 211

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1. Introduction

Transmission underground power cable circuits represent an important asset of the high voltage power infrastructure in most countries around the world. Due to the economic development and global population increase, power networks are expanding and therefore growth of the number of the installed kilometres is observed. Furthermore, transmission networks constructed as overhead lines are extended or reconstructed by new links. When overhead line installations cannot be used because of social or technical reasons, the underground transmission cables are an alternative solution. This ongoing process of continuous installation of the cable circuits is accompanied by the increased expectation on the quality assurance of the installation processes. Due to the fact that the existing procedures for after-laying testing reflects only the minimum requirements on safe operation, the after-laying procedures should be revised and updated with the newest best-practice applications including users’ experience.

The availability of energy transmission paths is related to system redundancy and failure rates of the particular cable link in the network. Failures can be of different origin, e.g. external (due to mechanical damage by third parties, installation related defects) or internal (due to cable system insulation defects or design problems). Recognition of failure symptoms is related to study of factors which may induce failure related defects in the transmission cable system. Knowledge about service conditions, e.g. load, ground temperature or cable construction is necessary to understand the relationship completely, e.g. between a particular cable type and a degradation process. Threat of functionality loss, outages, penalties for the delays in energy delivery paid to customer and repair costs are major consequences in case of unexpected failure of transmission systems. Environmental pollution, e.g. oil leakages from oil filled cables in case of external damage to the cable lead cover, also generates costs as a consequence of cable damage. Thus, to prevent asset failures, network operators are forced to implement, as a part of preventive maintenance, reliable assessment “tools” to check the condition of the most strategic transmission links and make the prioritisation of the assets that are in the worst condition and therefore are unreliable [1, 2, 3].

Besides visual inspections of, e.g. oil pressure/leakages or loss of continuity of external sheaths performed as a part of periodical maintenance of transmission cable circuits, technical data concerning transmission systems have to be collected by on-site testing and diagnosis. Such methods are used for verification of the cable insulation quality in different life stages. It is achieved through on-site tests and diagnosis where different cable insulation problems, e.g. partial discharge activity due to an internal insulation defect or increased dielectric losses can be detected, recognised and localised. Respectively, the service situation of cable circuits and tasks set for the particular situation can be divided as presented in Figure 1.1.

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Figure 1.1: Approaches to maintaining power cable reliability.

1.1 (E) HV Power Cables-underground transmission network

Underground transmission power cables are insulated systems designed to transport large amounts of energy in the range of hundreds of MVA (Mega-Volt-Ampere) at different voltage levels (Figure 1.2) having low losses and high reliability level. Cables represents 30 percent lower power losses than overhead lines at high circuit loads [4]. Typical European underground transmission networks operate with three-phase alternating current (AC) at different voltage levels. The standard power frequency in, e.g. USA/Canada and some countries of the Middle East is 60 Hz, while in Europe it is 50 Hz. Nowadays two types of transmission cables are mostly utilised for HV and EHV: cables with laminated insulation, e.g. Self-Contained Fluid Filled (SCFF), High Pressure Fluid Filled (HPFF) and cables with extruded polymeric insulation, e.g. cross-linked polyethylene (XLPE). A survey [2, 3] shows that more than 90% of the cables installed worldwide between 2000 and 2005 were XLPE cables at voltages below 220 kV. For operating voltages above 220 kV, SCFF cables represented 40% of the installed cables. The prevailing trend is an increased application of XLPE cables in particular for EHV voltages [2]. Another survey presented for example by Polish TSO-PGE Dystrybucja shows that since 2010 newly installed HV transmission power cables in the range of 110 kV in Poland consist of XLPE cables only [5].

Figure 1.2: Power Cable network voltage ratings e.g. in The Netherlands. Rating borders for some of countries can differ and in many countries HV transmission range starts at 30 kV, 50 kV or 66 kV.

[*]-Highest voltage rating for transmission cables so far (2015).

A transmission power infrastructure represents major capital investments. In 2006 almost

Low Voltage Medium Voltage High Voltage Extra High Voltage

1kV 33kV 50kV 66kV 110kV 150kV 220kV 800kV [2] Un

Distribution Transmission

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transmission networks in the future (Figure 1.3). This fast growth in cable lengths is also visible for the submarine cables where more and more extruded cables are used as presented in Figure 1.4.

Figure 1.3: Increase (per km) of installed underground power cable in the range from 110 kV to 219 kV between years 1996-2006 [2].

Figure 1.4: Installed kilometres of underground and submarine EHV power cables (AC and DC) in the world (transmission range 220kV-800kV) [6].

According to the 2016 year global market research report regarding HV cables entitled “High Voltage Cables & Accessories Market by Type (Overhead, Underground, & Submarine), by Overhead Products (Conductors, Fittings & Fixtures), by Underground and Submarine Products (XLPE Cables, MI Cables, Cable Joints, & Cable Terminations), and by Region-Global Trends & Forecasts to 2020”. The HV cable market is supposed to grow from $29 billion in

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2015 to $39 billion by 2020. Continuous growth of the underground transmission networks is triggered by three major factors of influence:

 Increase in human population and as a direct result, growing energy demands in densely populated areas.

 Development of polymeric insulating materials and technologically advanced HV cable system constructions and in future e.g. superconducting cables which allow upgrading the capacity and reliability of the energy transport.

 High reliability e.g. 99.96% of service availability is claimed by international statistics regarding 380 kV underground cable circuits (discussed in the next paragraph) [3]. The transition from laminated to extruded power cables since the end of 60s’ has resulted in continuously growing number of installed extruded transmission cables systems. As a result, production of laminated cables has gradually decreased. Extruded systems are cheaper and faster to produce (no impregnated medium). Quick installation and lower maintenance costs of extruded cables make transmission cables more and more affordable in terms of costs than overhead lines. In 2014, the ratio of the installation costs (per 1 km) between underground cables (HV range) and overhead lines was 10 to 3 (more than 3x higher cable costs) depending on the urbanisation level of the installation area [6].

Furthermore, the continuous development of technology-advanced transmission cables, e.g. HTS (High Temperature Superconductive Cables) allows for energy transmission with lower total losses than in case of traditional power cables and overhead lines. Application of extremely low loss insulating systems for regular high and extra high voltages is still being tested, however the results obtained in HV laboratories seem to put forward this trend in upcoming years for power cable applications [4].

Lastly, part of the overhead transmission lines, which were installed in the so called “country side” 40-50 years ago, are nowadays part of the populated towns and urban areas. In most of the cases, safety and public concerns forced the conversion to the underground transmission cables. The liberalisation of energy trading between the countries is also one of the factors leading to bulk power transmission over large distances. In case of highly industrialised heavy populated countries, e.g. Germany, The Netherlands, UK, Sweden, and Denmark, it is feasible only with the application of underground transmission cables.

1.2 After-laying testing and condition assessment of

transmission cables

The survey on transmission cables reliability presented in CIGRE TB 379 [3] shows that in the circuit length of 40 km, the number of cable failures during the nominal 40 year of service life would be approximately 4 for HV/EHV SCFF cables, which would mean 1 failure every 10 years. For HV/EHV XLPE cables the expected failure rate would be 1 failure every 20 years. Expected failure rates over 40 years for a typical transmission cable configuration consisting

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occurs, depending on the type of insulation, a repair can take up to several days or weeks. Duration of the outages varies widely, depending on the circumstances of the failure, availability of parts and skill level of an available repair personnel. A typical duration of e.g. gas-pressurised cable outage is 8 to 12 days [4]. The duration of HV/EHV XLPE outages is 5 to 9 days. A repair of a fault in SCFF/HPFF system is estimated to be from 2 to 9 months, depending on the extent of the damage [3, 4].

The reliability can be lower at the beginning of service life influenced by failures due to e.g. installation errors, which represent so called “infant mortality” period. At the end of service life, failures are related to ageing which represent a “wear-out” period. The “bath tube curve” reflects the average failure behaviour of a cable population (Figure 1.5). Changes over lifetime are showing that the failure rate of transmission cable is not constant over the life cycle.

Figure 1.5: Bath-tube curve describing HV/EHV equipment reliability and failure rate during service life [2, 3].

The transition from stage 1 to stage 2 depends greatly on the quality of cable manufacturing and the quality of the installation and service conditions, e.g. over-voltages. The stage 1 does not necessarily to be present, if there are no failures in the early lifetime. The transition between stage 2 and stage 3 is not fixed and it is difficult to predict or estimate. Generally, power transmission cables have a designed life time of around 40-50 years [2, 3, 4], however due to different service conditions and load profiles, it is rather difficult to provide the exact values. The progress of insulation ageing (deterioration) can be different even for the same cable type installed in different conditions, e.g. different climate.

One possible way to access the condition of power cables is on-site diagnostic tests [19]. The testing consists of several different on-site tests performed at different stages of service life of power cables. Figure 1.6 presents on-site tests, which are intended to reject faulty components or defective parts of the circuit after installation, assembling or repair, e.g. HV withstand test. There are also tests that give an indication of the insulation condition after service life time, e.g. after 40 years. The latter tests should have no impact on cable insulation and are called diagnostic tests.

Influenced by increasing number of new circuits

Influenced by increasing number of serviced aged circuits

F ailur e ra te Time Low “constant” failure rate

Beginning of life End of life

R

eliabilit

y

Stage 2 Stage 3 Stage 1

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Figure 1.6: Basic overview of the on-site tests performed on transmission power cables to assess their condition.

1.2.1 New cable systems

Early cable failure occurrence is prevented by high quality of the workmanship on-site. In order to check the on-site installation, the acceptance voltage withstand testing is performed on the newly installed cable systems. During on-site tests, both cable insulation and cable accessories are tested at an externally applied over-voltage condition just before putting them into operation [7, 8] (Figure 1.7). The test intends to reject any faulty component of the power cable system. The test is called successful if no breakdown was registered in any part of the cable system during testing time. Failure statistics of new cross-linked polyethylene (XLPE) cables at different voltage levels show that early failures occur during the first three years since the initial operation, 80% of these failures are related to local installation defects in cable joints and terminations [7]. Experience presented in [7] shows that 12% of all on-site acceptance tests on newly installed circuits resulted in breakdowns mostly in cable joints and terminations. The majority of problems occur due to poor workmanship during assembling, e.g.:

 Wrong assembling measures.  Assembly cleanliness.

 Wrong materials or tools used.

 Missing or wrongly applied electric field distribution elements of joints and terminations, e.g.: spacers, fillers,

semi-conductive materials, insulating tapes or defective deflectors.

Condition assessment of

transmission power cables

Installation test

(condition check of the cable parts after transportation, and laying)

Acceptance/after-laying test

(condition check of the complete cable system after accessories assembling)

After repair test

(condition check of the complete circuit after replacement of faulty component)

Aged circuits

e.g.: XLPE, SCFF, HPFF, GP, PE

New circuits

e.g.: XLPE

Single diagnostic test

(condition check of the cable parts and cable accessories after time in service)

Periodic diagnostic test

(condition check of the complete circuit in the time intervals e.g. every 1 year)

After repair test

(condition check of the complete circuit after replacement of faulty component)

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a) b)

Figure 1.7: Example of on-site testing of newly installed HV power cable circuits: (a) continuous AC voltages by resonance test system [ref. IPH Berlin website], (b) acceptance test using a damped AC (DAC)

system.

The trend in Figure 1.8 shows, that there is a high failure rate of new XLPE cables in the range of 60 kV to 219 kV, and the highest number of failures appears during the first 3 years from installation. Assuming that cable systems were tested properly after installation and have passed the tests successfully during the commissioning process, there must have been some hidden defect(s), which had not been detected by the applied testing procedures.

Figure 1.8: Trend in internal failures for XLPE HV power cables at different nominal voltage ranges [3]. These defects could have developed slowly and resulted in failure during operational service in the first year(s) of operation as it is reflected by the statistics in Figure 1.8.

1.2.2 Service aged cable systems

Condition assessment (evaluation) of cables in service can be described as activities focused on measuring or observing the key condition indicators (diagnostic parameters) of the primary cable components such as cable insulation. Comparing these measurement results to the existing standards and knowledge helps provide information about actual cable condition. Condition evaluation may be continuous or periodic. Condition assessment techniques may not cover all failure modes and a cable may fail without warning. Therefore, the way of “examination” of aged power cable systems should be adequate to the particular situation and be non-destructive.

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Different approaches are used by power utilities, e.g. visual inspection of the cable circuits (if possible), Dissolved Gas Analysis (DGA) of oil for SCFF, HPFF cables, dielectric loss estimation or analysis of the polarisation-relaxation phenomenon and counting the oil leakages (Figure 1.9).

Figure 1.9: Failure rates in SCFF (oil) transmission cables [9].

More detailed information about different diagnostic methods can be found in references [9, 10]. Condition assessment methods of cable systems depend basically on two factors: the type of power cable system and the cost/time necessary to evaluate the condition status of the cable circuit.

An investigation done at several power utilities, experienced in service life management of existing AC underground cables, shows that on-site condition assessment of aged cable circuits can help to discern cables, which are qualified for e.g. replacement and cables which can be still used without the risk of energy transmission interruption [3]. Around 51% of the utilities in the survey have introduced replacement programs for aged self-contained fluid filled (SCFF) transmission cables because of oil leakages. Oil leakages create energy delivery interruptions and are strictly related to the age of a cable (Figure 1.9). In this particular situation a cable fault is understood as any interruption (e.g. lower oil pressure due to the leakage), which puts the cable out of service. It is important to note that these interruptions are not always related to insulation breakdowns. In the investigated utilities, decisions about SCFF power cables replacements were mostly made based on the age of the assets and the number of oil leakage failures in a single cable link per year. Condition assessment results (cable diagnostics data) were taken into consideration in 50% of total investigated replacements and were considered an effective decision supporting tool [3]. In a considerable part of the investigated SCFF cable population on-site inspections and diagnosis confirmed slow ageing of HV terminations and cable insulation and no serious internal defects, which

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A different approach applies in maintaining the reliability of assets refers to aged gas-pressurised cables. In this type of cable, there is no visual evidence of the failure, such as oil leakages, and the general number of failures is relatively low in comparison to e.g. SCFF circuits. For example, the population of 150 kV external gas pressurised cables in The Netherlands reaches an average age of a single cable section of 30 years [11]. The study of the failure type and failure occurrence rate shows that, the character of the failures in gas-pressurised cables can vary from almost “random” occurring failures to distinctive ageing process related defects. An example presented in Figure 1.10b shows failure records of the gas-pressurised cable population in The Netherlands up to 2009. The total numbers of failures of this population is relatively low, i.e. 5 internal insulation failures of the cable per year at different ages.

Evaluation of the actual condition could be difficult when based only on the statistically calculated failure probability due to the low number of failures in this case. In The Netherlands high voltage some power network operators apply non-destructive diagnostic tests to assess insulation of gas-pressurised cables, e.g. damped AC (DAC) testing and PD diagnosis, dielectric losses measurements or dielectric spectroscopy. The obtained diagnostic parameters are sensitive to both thermal and electrical processes in the cable insulation.

Figure 1.10: a) Trend in internal failures for Self-Contained Oil-Filled (SCOF/SCFF) HV power cables at different nominal voltage ranges [3]. b) Trend in internal failures for 110 kV and 150 kV

gas-pressurised power cables in The Netherlands-situation till 2009 [11].

The increasing number of repairs on aged cable systems also forces the use of an effective non-destructive solution to assess the transmission cable status and check the quality of the

a)

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repair. Repair costs of transmission links can be very high, and the repair itself can be time consuming (depending on the type of cable and type of failure), when failure occurs. After repair, the cable system should be tested to check the repair quality and to ensure reliability of the repaired section prior to energising. The testing method should be selected in such a way, that the remaining life-time of the remaining part of the cable system (healthy part) should not be threatened [8, 10].

a) b)

Figure 1.11: Different approaches in condition assessment of service aged power cable system components: a) Visual inspection of 29 years old 50 kV cable terminations compartment (Coq type), b) Example of on-site off-line PD and dielectric loss non-destructive diagnostics on 220 kV, 20 years old

power cable with damped AC (DAC) system.

1.3 On-site testing methodology

The quality of cable system’s components has an impact on the availability of the electricity supply. To obtain full information about the technical status of the cable system, it is important to answer the following questions:

 What are the conditions of cable components, e.g.: terminations, joints, cable insulation?

 Is there any degradation process visible in the cable system components?

 Can the particular components of the cable system withstand service conditions with minimum risk of breakdowns?

 Are there any threats of potential failures in near future, e.g. next year or in five years, in any of the cable components?

 Will over-voltage service conditions like, e.g. switching surges or short-circuits, cause deterioration or defects, which can be a threat for the cable reliability?

To answer these questions, various types of on-site tests can be applied to power cables, e.g.: low/high voltage, on-line/off-line tests, electrical/non-electrical tests or destructive/non-destructive test. Detailed information about various testing methods for HV/EHV cables can found in [10].

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PD inception voltages of some defects can be above nominal voltage U0, so these can be only detected during the overvoltage condition. Thus, PD monitored voltage withstand tests at elevated voltages can detect:

 PD related defects.

 Defects that are sensitive to the withstand test duration and are visible after some time after test starts.

 Defects sensitive to integral insulation changes caused by e.g. moisture.

This thesis focuses on three testing approaches, being on-site voltage withstand tests, on-site off-line partial discharge (PD) diagnosis and dielectric loss on-site estimation.

Voltage Withstand Test

HV on-site withstand tests are used for [7, 8 , 10]:

 Commissioning of the equipment on site to demonstrate that transportation, installation and erection have not caused any new dangerous defects in the insulation.

 Checking the quality of repair of the equipment and confirming that all dangerous defects in the insulation have been eliminated.

 Diagnostic purposes to demonstrate that the insulation is still free of dangerous defects and the life-time expectation is sufficiently high.

The application of an overvoltage to the insulation is the oldest method of qualifying a cable for service conditions. This test is related to the nominal voltage (U0) or above nominal cable voltage (over-voltage) conditions. This simple test can be used to test whether the cable’s insulation holds the required voltage with a duration specified in applicable standard, e.g. according to IEC standard 1h. at 2.0xU0 for 110 kV cables. National standards, e.g. the NEN 3630 Dutch Standard, requires 10 min test at 2.5xU0 for the same cable. So, there is a world-wide discussion whether a higher voltage or a longer test duration is more effective for qualifying newly installed cable systems. Moreover, tests based only on a breakdown criterion are considered insufficient in order to reliably qualify the tested cable [7, 8]. An external voltage source is necessary for the voltage withstand test to energise the cable capacitance. For HV withstand testing two types of AC voltages are utilised nowadays [4]:

1. Continuous AC voltage (AC). 2. Damped AC voltage (DAC).

For continuous AC, the capacitance of the cable produces high power requirements for the test power generator to compensate losses during cable resonant voltage testing. For continuous AC voltage, a mobile resonance test-set is used nowadays with a variable test voltage frequency of 20-300 Hz. Another type of withstand test is the so called “soaking test”. This test is performed on new circuits or after-repair circuits at nominal voltage (50/60 Hz power network supply) obtained by connecting a new circuit for e.g. 24 h to the network without load [4]. The test is called successful if no breakdown occurred in any of the cable system components. However, such tests prove that this type of withstand test is not efficient as many installation related defects have partial discharge inception voltage (PDIV) above nominal voltage U0 [7, 8]. Since 1999, the damped AC (DAC) systems have been used for

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testing and diagnosis [12, 13]. Since 2015 the DAC method has been the standardised method by IEEE400.4 [14]. This method can provide a damped AC overvoltage withstand test with a fixed number of multiple DAC excitations. A single DAC excitation is a process which consists of:

 The charging period ranging from a few seconds up to a few tens of seconds (the duration depends on the capacitance of the tested cable).

 The switching period (few microseconds).

 Damped AC voltage, which is obtained through the resonance effect between cable capacitance and fixed system inductance.

The DAC frequency depends on the system inductance and the cable capacitance (given by cable parameters and cable length), the duration of the oscillation (range of several hundreds of milliseconds) depends on the damping effect of the cable. Withstand tests executed with DAC voltages can be performed simultaneously with PD diagnosis. In this way, monitored withstand tests can be executed on the test object. Diagnostic parameters characteristics as PD amplitude, intensity and location are used to qualify (assess) the circuit. In particular, partial discharges are detected according to IEC 60270 standard and evaluated. In such a way, the real condition of the cable insulation and accessories can be assessed by measuring the occurrence of PD related defects. This information is very important as the insulation breakdown can occur later during cable energising or a long service operation, but not necessarily within the withstand voltage test duration of e.g. 1 hour. A DAC voltage withstand procedure involves PD measurements during:

 Increasing the DAC test voltage level up to the maximum test voltage level of e.g. 1.7-2.5xU0 as some defects have PDIV at higher test voltages.

 The over-voltage withstand test to assess the effect of the maximum voltage and time.

 After the withstand test to confirm non-destructive character of the just performed DAC voltage withstand test.

As a result, data which are obtained during DAC voltage testing provide the following information:

 Effect of over-voltage condition (breakdown yes/no criteria).

 Presence of PD related and breakdown related defects in cable and accessories.  Characteristics such as PD amplitude, PD intensity, PD occurrence, PRPD patterns.  Location of the PD sources in the cable (using Time Domain Reflectometry analysis).  Information about the non-destructive character of the performed withstand test. Non-monitored vs. monitored voltage withstand test

Due to different purposes of withstand testing at different stages of cable service life or due to different possible defects present in insulation and accessories, the following aspects regarding voltage withstand testing have to be considered prior to test execution:

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 Non-destructive evaluation of the condition of the aged cable systems based on withstands tests.

Such tests consist of a combination of measurements of diagnostic parameters such as partial discharges and dielectric losses. In this mode, besides the yes/no criterion, additional criteria are considered as diagnostic parameters, e.g.: PD location, PD level, PD intensity and PD pattern characteristics observed while increasing the test voltage and the voltage withstand test itself. The test can be terminated, e.g. if PD parameters fall out of the specified safe value ranges. It is an especially important aspect for “mixed” cables, where in one cable link a very old part and a new part of cable coexist as a consequence of repairs. Table 1.1 shows major differences between monitored and non-monitored voltage withstand testing.

Table 1.1

Differences between monitored and non-monitored voltage withstand test.

on-site non-monitored withstand test on-site monitored withstand test

Process in which the behaviour of the newly installed cable system under electrical field stress is investigated (voltage withstand test) with application of external voltage source.

Applicable voltage sources: AC/DAC

Over-voltage: Yes

Testing is based on the application of electric stresses with a goal to indicate with pass/fail criterion the quality of the whole cable system being under the test and qualify it to service operation. Voltage and test duration depends on the type of the test object (voltage rating) and the recommendation used, e.g. IEC standards for on-site testing of HV power cables.

Diagnostic parameters: No

Test criteria: breakdown during the test

Experience in diagnostic data analysis :No

Process in which internal cable defects and dielectric insulation properties are evaluated under electrical field stress generated by external voltage source.

Applicable voltage sources: AC/DAC

Over-voltage: Yes, but in case of service aged circuits it is not obligatory due to diagnostic parameters.

Diagnosis is based on the application of electric stresses in combination with selected diagnostic parameters to indicate/localize/identify a defect. Voltage and test duration depends on the cable condition and its failure history. Based on the interpretation of the diagnostic, information about the defect(s) impact on power cable reliability can be obtained. Diagnostic parameters: Yes

Test criteria: breakdown and diagnostic parameters like: PD parameters and PD location, dielectric loss, leakage-currents, insulation resistance.

Experience in diagnostic data analysis :Yes

Results shown in [15, 16, 17] on on-site condition assessment of service aged HV/EHV power cables, show that the PD measuring method is reliable in retrieving information about the ageing status of the insulation and cable accessories. This approach is getting more and more popular together with the standard voltage withstand test and is called “monitored voltage withstand test/diagnosis” even though on-site PD measurements are actually not mentioned by IEC standards and only described in IEEE 400.4 (2015). According to the data presented by IPH Berlin in 2007, the total number of after-installation tests with PD measurements was

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clearly increasing [15], for example in 2000 the company performed 25% of the total number of the acceptance tests in parallel with PD diagnosis, whereas in 2006 75% of the total acceptance tests were accompanied by PD measurements. In 2015, IPH Berlin claimed that almost 90% of the total performed acceptance tests in Europe were monitored by parallel PD measurements.

On-site PD diagnosis

Two approaches to on-site PD measurements are used nowadays (Figure 1.12): conventional (classical) PD detection (according to IEC 60270) and non-conventional PD detection. The advantage of PD measurements in compliance with IEC 60270 is a specified calibration procedure which ensures reproducible and comparable PD test results. Measurements can be performed with different measuring systems which give the reading output in pico Coulombs [pC] of the measured apparent charge. However, the main drawback of conventional PD measurements is that the signal/noise ratio is strongly reduced by the limitation of measuring frequency below 500 kHz. Measurements with conventional systems operate in the frequency range below 1 MHz. Narrow-band measurements in the 9-30 kHz are characterised by a centre frequency between 50 kHz and 1 MHz, whereas wide-band measurements are in the range <100 kHz-1MHz>. Conventional systems utilise a coupling capacitor connected in parallel to the test object in order to measure the charge transfer and voltage drop occurring at the circuit during the discharge.

Figure 1.12: PD detection frequencies range for conventional and non-conventional PD detection. Secondly, non-conventional detection, e.g. radio frequency (RF) technique detects the transient electromagnetic phenomena generated by charge displacement in the discharge area (locally). PD pulses are characterised by rise times in the nanosecond range. Therefore, the resulting frequency spectrum may cover several hundreds of MHz and more. This high frequency spectrum is well detectable by means of inductive and capacitive sensors. The measuring system can be applied locally on power cable accessories like joints and terminations. Figure 1.13 presents a basic configuration where PD sensors are installed around the ground connections of the accessories. With non-conventional PD detection it is not possible to calibrate the system and test object in accordance to IEC 60270, therefore PD readings are displayed in volts, or more often in mV. Selecting centre-frequencies with a proper transfer can be done by determining the transfer function of cable joint or termination in combination with the HF/VHF/UHF partial discharge detection system. After certain sensitivity of the measuring system is obtained, the performance check can be done to verify the detection on a particular test object in specific on-site conditions. This method can be utilised both for on-line and off-line PD measurements.

Narrow band HF 9 kHz 30 kHz 3 MHz IEC 60270 Conventional PD detection IEC 62478 Non-conventional PD detection 300 MHz 3000 MHz UHF Wi de band 1 MHz 30 MHz VHF

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Figure 1.13: Principles of voltage testing and PD measurements a) conventional (off-line) PD detection in Pico Coulombs [pC], b) un-conventional PD detection (on-line/off-line) in volts [V].

On-site dielectric loss diagnosis

Dissipation of energy in insulation under electrical stress is caused by several dielectric phenomena. The most important are [18]:

 Conductive losses caused by finite bulk resistance of the insulation and other leakage currents.

 Polarisation losses arising from dipole orientation under the applied field.

 Interface losses between materials with different dielectric constants (or relative permittivity).

 Partial discharge (PD) losses.

These processes contribute to increase of dielectric losses, which can be measured as the dissipation factor or tan δ. This diagnostic parameter can be used to measure integral insulation degradation. In the past, the dielectric losses could be measured only with laboratory methods, e.g. Schering-Bridge, which were very sensitive to temperature and moisture and therefore not usable in the field. Other solutions to measure dielectric losses are based on the analysis of gas content in impregnating/insulating medium, e.g. DGA (Dissolved Gas Analysis). This method is applicable mostly for SCFF or HPFF cables and oil filled cable terminations. The content of gases in oil samples, e.g.: methane, butane, which are related to certain ageing phenomena and can be determined in the laboratory. Assessment procedures involve on-site acquisition of a small sample of oil during the inspection of the oil reservoir tanks for laboratory analysis, therefore this method can be considered as “semi-on-site”.

Estimation of dielectric loss from DAC voltages

In case of DAC voltages test method, the estimation of dielectric loss parameter (dissipation factor) is based on the calculation of the DAC test voltage attenuation coefficient. During the on-site voltage testing of HV cables the obtained damped sine wave is characterised by a

Joint Joint

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certain damping factor (Figure 1.14). It is the attenuation factor β that depends on the cable impedance and the internal DAC system resistance (determined during factory calibration).

Figure 1.14: Attenuation coefficient calculation principles.

For the calculation of the attenuation coefficient two peaks (positive or negative) are taken from the obtained DAC voltage. When combined with the corresponding time instants, it will give the value of the attenuation coefficient. The calculated value of the attenuation coefficient β, together with the total equivalent internal system resistance R1 (described in details further in this thesis), oscillating frequency ω and inductance L will give the value of the dielectric losses called DL in the tested cable according to equation below [14].

1 2 R DL L      [1-1]

1.4 Asset management for transmission cables

Asset management (AM) refers to certain strategies to obtain the required reliability of the power network [1]. In general, AM decisions for transmission power cables focus on cable systems reliability. One of the goals is the constant low failure rate in conjunction with economic, environmental, societal and technical factors. In particular, five main processes for AM decisions can be specified to achieve this goal:

Refurbishment-replacing some components of the old system to achieve enhanced assets reliability, functionality and availability.

Upgrading-replacing the aged cable system components with new ones to restore to “as new” condition.

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Replacement-complete change of the aged circuits, e.g.: old oil filled cables with extruded cables to achieve, e.g. lower pollution of the environment, more capacity.

Repairs-replacement or fixing of small portion of the circuit that is faulty and may influence cable reliability.

Maintenance-with minimal cost keeping aged assets running.

The asset management decision process regarding servicing aged systems is usually dictated by a long-term replacement policy of the oldest or the cables in the worst condition (Figure 1.15). Power utilities have to select which particular section or complete circuit should be replaced in the first place. From a technical point of view, the prioritisation of assets’ health in terms of: a critical, semi-critical or safe asset is based on:

 Actual condition of the assets-analysis of different diagnostic parameters, e.g.: dielectric loss and PD or a combination of both.

 The criticality of the equipment in the network.  Failure rate.

 Expected remaining life time at particular service conditions.

Figure 1.15: Impact of the AM decision on failure rate (life time) of power cables.

All information inputs should be correlated and investigated in order to specify the optimum sequence of replacement/refurbishment for the individual cable section. In order to make a judgment whether an asset has reached the end of its useful life, it is important to set up the acceptable limits for particular service life stages. Once the asset condition falls outside such limits, action is necessary as presented, e.g. in Table 1.2. Furthermore, the ageing models that indicate the degradation rate of cable insulation can be also used to support final decisions. Ageing tests on cable samples taken out of service or cable system components reaching breakdown or the moment of harmful changes in the insulating material, should provide the reference values to support the on-site condition assessment on the similar type of cable/insulation type and to understand the ageing processes.

AM decisions have to

be taken in this

period to extend

service life of power cables

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Table 1.2:

Example of condition indexing (CI) and condition related AM decisions [1].

The technical impact on final AM decisions is the result of an interaction with economical and societal aspects, which are different for each company. Some power utilities’ strategies are shaped by public service, minimum cost; others may be shaped by maximum profit or parameters such as cash flow, return on investment etc. For others, public image may take priority [1]. The most critical (unreliable) asset will be weighted with those criteria important to the business policy. Therefore, condition data describing actual status are crucial for asset managers in making the final decision. Based on the information provided during periodic or condition based inspection, the actual condition of the tested cable and components can be used to schedule necessary maintenance activities and to determine the reliability of the particular asset in the total network configuration.

1.5 Objectives of present study

HV/EHV cables should be reliable under all operational conditions and stay defect free in service. For this reason, the testing system shall be designed in such a way that besides overvoltage test conditions, diagnostic parameters can be measured and evaluated.

The purpose of the investigation presented in this thesis is to develop a standardised uniform test procedure using DAC for both newly and aged transmission power cable systems in the field. Additionally an AM (asset management) decision support model will be developed based on both service and operational conditions of power cable service. To achieve this goal, research results from on-site tests obtained with DAC method in the field as well as in the laboratory. Accelerated ageing tests on oil-impregnated insulation will be compared and verified with the Arrhenius’1 formula. Results of both the laboratory and on-site test will be used to predict cable life time. On-site diagnosis on HV and EHV cables contained in this document are based on four years of practical research and diagnosis in the field of DAC voltages. All laboratory investigations on the detectability of assembling errors are conducted on real full size cables and cable accessories. Furthermore, the following research was performed:

Index Status Condition AM decision

A Very good

condition, stable

New or Aged: no defects or aging symptoms

Minimal maintenance – go for optimal availability and reliability

B Good condition

Aged: degradation observed, possible presence of harmful defects

No impact on the reliability – keep status B, against minimal costs and maintain optimal availability

C Stable, poor condition

Reliability decreased. Refurbish to status B or stabilise to prevent fast aging. Maintain optimal availability by periodical condition assessment

D Instable situation

Defect present, progressing aging

Very low reliability. Instable situation, end as quick as possible.

E End of life cycle

No operation recommended. Maintenance aimed to prevent environmental pollution and safe conservation. Extension of life time or availability is no issue.

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 Identification of the typical joints and terminations assembly defects that are related to poor workmanship onsite.

 Evaluation and verification of PD detection capabilities of the DAC method for different DAC frequencies that depend on the cable length.

 Analysis of the PD measurements results ranging from newly installed to aged cable systems in the field.

 Statistical analysis of PD detection and dielectric loss measurements performed on service aged oil impregnated paper insulated 50 kV and 150 kV gas-pressurised power cables.

1.6 Structure of the thesis

The research in this thesis is structured in the following way:

Chapter 1: Introduction to transmission power cables, on-site testing methodology and problem definition. With reference to the latest IEEE 400.4 2015 guide, discussion about off-line monitored/non-monitored voltage withstand testing. Applicability description of the two major diagnostics parameters used nowadays for on-site tests of HV/EHV power cables.

Chapter 2: Overview of the most important assembly defects in HV power systems which may lead to cable failure. Review of major installation related defects of extruded cable accessories and defects symptoms.

Chapter 3: Ageing-related defects in HV power cable systems and symptoms.

Chapter 4: Damped AC method: voltage generation, technical aspects, features, applicability, available diagnostic parameters.

Chapter 5: Investigation of installation defects in extruded cables. Laboratory investigation on detectability of different defects for different energising sources: continuous AC 50 Hz and DAC at different frequencies.

Chapter 6: Examples of on-site tests and diagnosis on HV/EHV power cables. Examples of condition assessment approach for newly installed and service aged transmission cables by on-site diagnostic tests. Procedures for testing, result analysis for the DAC method.

Chapter 7: Generation of the “insulation life consumption model” to support asset management’s decisions based on two different types of data:

 Operational cable parameters, e.g.: cable load, cable temperature  On-site diagnostic parameters: dielectric loss measured with DAC

method

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2. Transmission power cable accessory

installation and assembling related

defects

Cable installation and assembly failure refers to the situation when one or more cable system components such as cable joint or termination fails because it cannot withstand operational conditions. This happens as a result of local lower dielectric breakdown strength. Lower dielectric voltage breakdown strength is related to dielectric material condition.

“Most failures in paper insulated cables are due to third party damage and other exterior failure causes (67%), followed by oil leakage of SCFF cables (12%). Overloading is not considered to be a failure cause at all (0%). Regarding high voltage cables with extruded insulation, only 7% of all failures are due to ageing. Third party damage, water treeing and other causes such as poor installation, manufacturing and construction, rank higher with 65%, 13% and 15% respectively [9].

Based on the literature study and onsite experience, several most common assembling defects are explained in this chapter. Some of them are simulated in the HV laboratory and further investigated in chapter 5.

2.1 Accessory assembly

Installation and assembly defects are related to poor workmanship. Considerable amount of assembly defects result in high electric field distribution inside the cable accessory [3, 4, 17]. HV/EHV cable accessories that are not free of these defects should fail during over-voltage acceptance tests. However, sometimes a defect is not detected and causes a failure, e.g. a few months after installations. Our experience collected during acceptance tests (after-laying of new circuits) pointed out that mainly poor workmanship is responsible for cable system failures that occur during after laying testing. Investigation results presented in [7] confirm that “12% of the total number of performed tests resulted in breakdowns mostly in joints and

terminations during voltage withstand tests at maximum test voltage level or during voltage increasing. While some of the installation defects are intermediate failures, other develop slowly and need more time to develop into a failure under operational conditions”. Most of HV

and EHV cables have extruded polymer insulation (XLPE) nowadays. The examples of installation defects which will be presented in this thesis refer to accessories for this type of cable only.

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2.1.1 Assembling process

Assembling of cable accessories is a process where two cable parts are galvanically connected. Their integrity is ensured by application of cable joints, which are also called “splices”. Splicing or jointing of high voltage electrical cables is required in order to maintain the electrical continuity of the conductor and shields, as well as to maintain the insulation levels of the two electrical cables joined together. By jointing two separate conductors, insulation is also built up over the exposed conductors to provide insulation properties similar to those of the cable insulation. Cable shields are jointed to confine the electric field to the insulation [4]. Different types of joints are used, e.g. stop joints (prevent oil circulation in the cable in case of SCFF and XLPE cables connection), transition joints (connect different types of cable within one circuit), straight-through joints and cross-bonding joints. More information about joints can be found in [4]. Before the two cable parts can be “connected” they need to be carefully prepared prior to the jointing process. In general terms, such a process is called “cable peeling or cable preparation” and it is performed in compliance with standardised procedures and measures aimed for particular types of cables and joints. For instance, in the case of XLPE cables, the cable’s outer protection (cable sheath) as well as field-smoothing layer have to be removed during the peeling. The exposed conductive layers, e.g.: semi-conductive screen (semicon), outer and inner screens, produce a concentration of field. Due to the presence of tangentially distributed electrical field stress, electric field is built up in the area around the cable dielectric and the insulation boundaries [4]. Application of a joint (including grading elements) ensures not only proper electric field distribution in the area of galvanic connection, but also stable electrical and thermal conditions. Figure 2.1 presents the process of cable preparation prior to jointing (splicing) [4]. Solid dielectric insulated cables are jointed in one of the two ways:

1. Hand wrapping with insulation tape.

2. Slipping a pre-moulded cable splice on the joint/terminated conductors.

In case of hand wrapping, a highly-skilled cable joiner, is required. A cable jointer/splicer must select the proper tape for each stage of the insulation building process and make sure that no air voids or contaminations are present in the final joint.

Pre-moulded or slip-on cable splices for solid dielectric insulated cables are easier to install than for example oil-filled cables. This type of cable splice is designed and manufactured in order to provide the insulation levels and electric field distribution required for a given size of electrical cable. Therefore, the manufacturer is able to test the insulation level and electric field control prior to shipping. This type of cable splice can be installed in less than half of the time required for a hand-wrapped cable splice, although peeling and cable smoothing is also required. Assembly of cable accessories is also related to cable terminations (sealing ends). Here cable ends are connected to the electrical network system with different types of terminations, e.g. transformer sealing ends inserted directly into transformer body, outdoor terminations connected to overhead lines, GIS terminations (gas insulated and integrated within switch gear body). More information about terminations can be found in [4]. Cable termination functionality and internal construction can be compared to “half” of the construction of a cable joint. In case of a termination, the electric field distribution needs to be altered (distributed) in the same way, too. Field steering elements of e.g. polymer insulated power cable terminations have one deflector applied over the “peeled” edge of the

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